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Chapter 6 : Case Studies
UNIX
Dos
Windows 95
Windows NT
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UNIX
There are two main versions:
AT&T System V Release 4 (SVR4)
Originally developed by AT&T, now SCO
BSD (Berkeley Software Distribution)
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Supports various architectures
Structure varies
Supports preemptive multitasking
Multiuser environment - generally secure
Supports multithreaded applications
Protection/Security is high on modern versions
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Supports symmetric multiprocessing
Highly scalabe/portable to various systems
Many types/flavours of UNIX exist
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MS-DOS
Limited to x86 architecture
Simple Structure
Single User (No tasking, scheduling, priorities)
No protection
Monolithic structure
Old, outdated but still widely used (hardware and cost driven decision)
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Windows 95
x86 architecture
Layered
Single user
32 bit applications run in a preemptive, multithreaded, multitasking environment
16 bit applications (of Windows 3.x) run in shared memory space in a cooperative multitasking environment
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Virtual DOS Machine for DOS applications
“Plug and Play” - new OS “advantage”
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12. Virtual Machine Manager subsystem File Management subsystem
Windows 95 Architecture
System Virtual Machine
Win32
app
System
services,
KERNEL,
USER,
and
GDI
Win16
DOS app
DOS Virtual Machines
Ring 3
Ring 0
Virtual Machine Manager subsystem
File Management subsystem
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VMM (Virtual Machine Manager) and file management subsystems run in ring 0 (the most privileged level of the four-ring Intel 386 protection model). This implies these two modules have unrestricted access to the whole system.
VMM provides memory management, scheduling and DOS Protected Mode Interface (DPMI)
VMM also includes all Virtual Extended Drivers (VxDs) to virtualize hardware devices.
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The system Virtual Machine runs in ring 3 (The only other protection level used by Win 95. Ring 3 is the least privileged, best isolated level)
All applications and core dynamic link libraries (DLLs) - KERNEL, USER, and GDI execute in the VM
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15. Multitasking & Multithreading
Multiple concurrent threads
Time is allocated on a per-thread basis
DOS VMs and VM address space shared by Win16 applications constitute one thread each
Each Win32 apps represent at least one, and possibly two or more, threads
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Scheduling
The scheduler allocates time based on execution priority, taking into account factors such as whether the thread belongs to a foreground or background process and how long it has been since the thread was last run
Win32 and DOS apps are preemptively multitasked.
Win16 apps are cooperatively multitasked within the time slice allocated.
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17. Desynchronized Input Queues
An input queue is where hardware generated messages such as mouse clicks and keystrokes are stored for app retrieveal
Win95 assigns each application its own input queue
In Windows 3.x there is only one input queue so a misbehaving process may delay all other applications
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18. (Symmetric Multiprocessing) SMP
Not applicable to Win95.
Only in NT versions
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File System
Windows 3.x Oss use DOS services to perform file I/O.
Win95 uses a 32-bit (16-bit for Windows 3.x) file system VFAT (Virtual FAT)
VFAT is compatible with FAT volumes
VFAT permits long file names (up to 255 characters)
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Windows NT
32 bit preemptive multitasking OS
Supports different chip architectures (CISC and RISC)
Can work as a server or client (workstation)
Has the same GUI as Windows 95
Modular style
Multiuser, multitasking
Supports threads as well as classical processes
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Support SMP (Symmetric Multiprocessing) (1 to 32 processors)
Designed with security in mind
Supports distributed processing - networking is a core component to the OS
Supports RPCs
Very robust (reliable)
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The HAL (Hardware Abstraction Level) makes porting very easy
Supports non-NT applications through environment sub systems - meaning it can run Win32, OS/2 and POSIX applications (supports multiple personalities)
NT style OSes are the way of the future
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24. Comparative OS Characteristics
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References
Modern Operating Systems, Andrew S. Tanenbaum, Prentice-Hall,
Operating Systems (Second Edition), William Stallings, Prentice-Hall,
Local Area Networks : A Client/Server Approach, James E. Goldman, John Wiley&Sons, 1997
Your Next Operating System, PC Magazine, September 26, 1995, pp
Windows 95 And Its Competitors, PC Magazine, May 16, 1995, pp
Chicago: Under Construction, PC Magazine, April 12, 1994, pp
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